The present invention relates to an exhaust gas purification device for an internal combustion engine.
The present applicant has already proposed an exhaust gas purification device for an internal combustion engine, in which a NOx absorbent is disposed in an exhaust passage of an internal combustion engine to absorb NOx (nitrogen oxide) in the exhaust gas when the exhaust gas flowing therein has a lean air-fuel ratio and to release the absorbed NOx when the oxygen concentration in the exhaust gas flowing therein has decreased, so that NOx in the exhaust gas is absorbed by the NOx absorbent while the engine is operated at a lean air-fuel ratio (see International Unexamined Patent Publication WO93-25806). The exhaust gas purification device disclosed in this publication is equipped with an estimation means for estimating the amount of NOx absorbed by the NOx absorbent in order to monitor the NOx holding amount in the NOx absorbent at all times during operation. When the NOx holding amount reaches a predetermined value, the oxygen concentration in the exhaust gas flowing into the NOx absorbent is lowered to release the absorbed NOx from the NOx absorbent and to purify the released NOx by reduction with reducing components such as unburned HC and CO in the exhaust gas (in this specification, the operation for releasing the absorbed NOx from the NOx absorbent and for purifying the NOx by reduction is called xe2x80x9ca regenerating operation of the NOx absorbentxe2x80x9d). According to the exhaust gas purification device taught in the above-mentioned publication, the regenerating operation is executed every time the NOx holding amount of the NOx absorbent reaches a predetermined value, so that the NOx holding amount of the NOx absorbent will not increase excessively and that the NOx absorbent will not be saturated with NOx which it has absorbed.
When the regenerating operation of the NOx absorbent is executed every time when the NOx holding amount estimated during the operation of the engine has reached a predetermined value, however, there remains a probability in that unpurified NOx is released from the NOx absorbent at the start of the engine.
When the regenerating operation is executed every time when the NOx holding amount in the NOx absorbent has reached a predetermined value during the operation of the engine as done in the device taught in the above-mentioned publication, it may often happen that a considerable amount of NOx remains held by the NOx absorbent at the next start of the engine when, for example, the engine is stopped just before the NOx holding amount in the NOx absorbent has reached the predetermined value.
At a cold start of an engine, in general, it is accepted practice to effect the fuel increment for warming-up or the fuel increment for start-up by supplying fuel in an increased amount to the engine based on the engine temperature, so that the engine is operated at an air-fuel ratio (e.g., an air-fuel ratio of from about 12 to about 14) which is more than a normal air-fuel ratio for a predetermined period of time after the start. The fuel increment decreases with a rise in the engine temperature and is canceled after the engine has been warmed up. That is, immediately after the start, the engine is operated at a rich air-fuel ratio. As the engine is gradually warmed up, the air-fuel ratio approaches the stoichiometric air-fuel ratio. After being warmed up, the engine operates at a lean air-fuel ratio based on the operating conditions. Therefore, the NOx absorbent is exposed to the exhaust gas of a rich air-fuel ratio due to an increase in the fuel supply at the start of the engine.
In order for the NOx absorbent to exhibit its NOx absorbing and releasing action, the NOx absorbent must have been heated to a temperature in excess of an activating temperature (e.g., about 250xc2x0 C.) based on the kind of the NOx absorbent. When the NOx absorbent is at a low temperature, such as right after the start of the engine, therefore, no NOx is released from the NOx absorbent even when it is exposed to the exhaust gas having a rich air-fuel ratio.
With the NOx is absorbed in relatively large amounts by the NOx absorbent at the start of the engine, however, the absorbed NOx is released rapidly when the NOx absorbent is heated at a temperature in excess of the activating temperature after the start of the engine. As described above, the fuel increment after the start of the engine decreases with a rise in the engine temperature. When the temperature of the NOx absorbent has reached the activating temperature, therefore, the engine temperature has been raised correspondingly, and air-fuel ratio in the exhaust gas is not sufficiently rich.
When the NOx is released rapidly from the NOx absorbent in this state, HC and CO necessary for reducing the NOx become in short supply on the NOx absorbent; i.e., the NOx that is released may often be released into the open air without being purified.
Since the engine operating condition is not stable until the engine is warmed up after starting, when the engine starts with NOx being absorbed in relatively large amounts by the NOx absorbent, NOx may often be released without being purified from the NOx absorbent due to a change in the operating conditions. Besides, the amount of the NOx that is released without being purified increases with an increase in the amount of NOx absorbed by the NOx absorbent. When the NOx absorbent having a large maximum NOx holding capacity (capable of occluding large amounts of NOX) is used, therefore, NOx is released in an increased amount without being purified.
Further, in the device for regenerating the NOx absorbent every time when the NOx holding amount in the NOx absorbent reaches a predetermined value while the engine is in operation as done by the device taught in the above-mentioned publication, the timing for executing the regenerating operation of the NOx absorbent may become incorrect if NOx remains absorbed by the NOx absorbent when the engine that has been warmed up is shifted to the lean air-fuel ratio operation, in addition to the above-mentioned problem. That is, in the device taught in the above-mentioned publication, the NOx holding amount in the NOx absorbent is monitored at all times, and the amount of NOx held by the NOx absorbent when the engine is halted is known. Therefore, if the NOx holding amount at the next stop of the engine is stored in a nonvolatile memory or the like means, it will be possible to estimate the correct amount of NOx held by the NOx absorbent from the start of the engine based on the stored amount and, hence, to execute the regenerating operation at a correct timing. In practice, however, NOx may often be released from the NOx absorbent while the engine is not in operation, and the NOx holding amount in the NOx absorbent at the start of the engine may often become different from the NOx holding amount of when the engine was halted in the previous time. Therefore, if the NOx holding amount after the start of the engine is estimated based on the NOx holding amount of when the engine was halted in the previous time, a difference occurs between the actual NOx holding amount and the estimated value, and the timing for thee regenerating operation becomes incorrect, deteriorating the quality of the exhaust gas.
In view of the above-mentioned problems, the object of the present invention is to provide an exhaust gas purification device for an internal combustion engine, which releases nearly all of NOx absorbed by the NOx absorbent during the operation of the engine in the previous time and reduces NOx by reduction, in order to prevent deviation in the timing for releasing the unpurified NOx after the start and in the timing for executing the regenerating operation.
According to the present invention, there is provided an exhaust gas purification device for an internal combustion engine, comprising:
a NOx absorbent, disposed in an exhaust passage of the internal combustion engine, which absorbs NOx in the exhaust gas when the air-fuel ratio of the exhaust gas flowing in is lean, and releases the absorbed NOx and purifies it by reduction in a rich air-fuel ratio atmosphere; and
a NOx-releasing means which, after the start of the engine, operates the engine at a predetermined rich air-fuel ratio determined by increasing the amount of fuel supplied to the engine, so that NOx absorbed by said NOx absorbent is released and is purified by reduction until the engine is first operated at a lean air-fuel ratio after the engine is started.
That is, according to the present invention, the regenerating operation of the NOx absorbent is executed at a predetermined rich air-fuel ratio after the start of the engine until the engine is first operated at a lean air-fuel ratio. The rich air-fuel ratio is the one which is different from an ordinary air-fuel ratio at the start of the engine, and with which the whole amount of NOx that is released can be purified by reduction even when the NOx is released in relatively large, amounts from the NOx absorbent. Therefore, nearly the whole amount of the NOx absorbed by the absorbent is released from the NOx absorbent and is purified by reduction before the engine is operated at a lean air-fuel ratio, making it possible to prevent unpurified NOx from being released at the start of the engine. Irrespective of the absorbed amount of NOx of when the engine was last stopped, further, almost no NOx has been absorbed by the NOx absorbent at the time when the engine assumes the lean air-fuel ratio operation. This makes it possible to correctly estimate the amount of NOx absorbed by the NOx absorbent during the operation and, hence, to correctly operate the timing for the regenerating operation.
As described above, the amount of NOx absorbed and held by the NOx absorbent is decreased (or, preferably, decreased to almost zero) after the start of the engine until the engine assumes the lean air-fuel ratio operation. At the time when the engine first assumes the lean air-fuel ratio operation after the engine is started, therefore, the NOx holding capacity of the NOx absorbent can be increased nearly up to its maximum limit. When use is made of the NOx absorbent having a maximum NOx holding capacity (maximum amount of NOx that can be occluded) to absorb and hold, as much as possible, the whole amount of NOx produced during the operation of the engine, therefore, the regenerating operation is no longer required during the ordinary lean air-fuel ratio operation of the engine. The regenerating operation may be executed only after the start of the engine.